I love that "above some critical threshold" is listed like a mysterious or complex thing. It's the angle of repose, the angle that a material naturally sits at when you let it fall from a height and pile up. It might be, if things are very complicated, the angle of repose + cohesion, but then you're back at water-based theories again since water is the easiest way to remove cohesion and trigger failure.

I also really like that the experimenters managed to recreate a sand flow in their lab. Of course they did. The field of prior research involving laboratory sand flows is immense, especially if you start including the ones with tiny glass beads of carefully varied diameters instead of sand. The only problem is thioxtropy -- landslides are renowned for having material that exhibit viscosity inversely proportional to velocity -- which is not easily replicable in small-scale lab settings.

I'm not sure if this is a, "Physicists discover what geologists already knew" moment, or a "Journalists are puzzled by the mundane mysteries of science," or what, exactly, but if you want to learn more about landslides on Mars, check out geotechnical journals starting with Lucchitta 1978 (Bulletin of the Geological Society of America, v89, pg 1601) and work your way forward. As the lunar and Martian landslides discredited an entire set of excess mobility theories, they're very well described and discussed.

From the original blog post : "over this timescale, the Martian atmosphere has been too cold and thin for liquid water."

I read something like this frequently, and yet it is simply wrong and I wish people would stop repeating it.

Liquid water is not magic, but governed by physics. For there to be liquid water on Mars, all that is needed is that water be present, that the surface pressure be above the triple point of water, and that the temperature be above the freezing point. (Actually, this can be relaxed somewhat for brines and the like, but let's put that aside for the moment.) We know that Mars has water. What about the other two conditions ?

Much of the surface of Mars is above the triple point of water (i.e., at a low enough elevation that the surface pressure is higher than 611.7 pascals). Any low lying region is. The Viking 2 landing site is (some of the time) and the Phoenix landing site is (all of the time). The entire Hellas basin is, and it is highly likely to have liquid water at times (as the surface temperature there is warm enough during the day). Remember, peak surface soil temperatures on Mars can reach 27 C, even under current climate conditions.

Further, the atmospheric pressure on Mars varies greatly during its obliquity cycle, and it is highly likely that the entire planet (except for the high volcanoes of Tharsis) can support liquid water at times during each obliquity cycle. During those phases of the cycle, the atmospheric temperatures will be generally warmer, as well.

Now, this does not prove or disprove that these gullies are formed by water rather than sand, but you don't need unusually strong brines or geothermal vents to have liquid water on Mars (even though both of those probably exist as well), and it is quite reasonable to expect its presence in places, even under current atmospheric conditions.

The original press release did not say anything about "definitive" evidence of water. In fact it said:

"Certain tasks remain, according to the panelists. For example, a spectrographic analysis of the “white stuff,” to prove that it is definitely water. These might be carried out by the Mars Reconnaissance orbiter, recently arrived in order to replace the aging Global Surveyor."

So the original report said something looks like water. It isn't just a flow of dust.

And the current summary is WRONG. The new article also agrees that it is NOT simply flowing dust and sand.

"There is another possibility, however. Perhaps the gullies are caused by the flow of sand and dust. Similar gullies are known to occur on dunes on Earth but only when the angle of the hillside is above some critical threshold. The trouble with Martian gullies is that most of the hillsides are not steep enough for this process to occur."

The actual new proposal is this:"Their idea is that the gullies are formed when carbon dioxide in the ground sublimates, causing the sand to become fluidised."

This actually AGREES with the original report that said :

“These things appearing bright is extremely unusual,” said to NASA panelist Michael Malin, explaining why NASA believes the apparitions are water, not mere avalanches of dust. “In the past, the things we’ve seen are very dark this requires some kind of fluidizing agent.”

So.... rather than contradicting ANYTHING that the original press release said this new information is merely another possible explanation for the surface feature.

This brings us back full circle to:

"Certain tasks remain, according to the panelists. For example, a spectrographic analysis of the “white stuff,” to prove that it is definitely water. These might be carried out by the Mars Reconnaissance orbiter, recently arrived in order to replace the aging Global Surveyor."